The present invention relates to a method for cutting through a brittle material, especially glass, along a predetermined separating line, in which the separating line is heated with a heat radiation spot symmetrical to it, which has edge portions with an elevated heat radiation intensity and a temperature maximum at its rear end; the heated region is moved along the cutting line and/or the workpiece and a section of the heated cutting line is subsequently cooled. The invention also relates to an apparatus for performing this method.
Conventional methods for cutting flat glass are based on first making an extended scratch in the glass by means of a diamond or a cutting wheel in order to break the glass subsequently along the weakened region of the scratch by an external mechanically applied force. In this method disadvantageously particles (splitter) that can be deposited on the glass and can lead to further scratching there are released from the surface by the scratch. Similarly curved glass fragments are produced at the cut edge, which leads to an uneven glass edge and thus requires considerable after-working expense. Furthermore the micro-fractures arising during the scratching lead to a reduced capability to take mechanical stresses at the cut edge, i.e. to an increased danger of breakage.
Some progress toward avoiding both splitter and also glass fragment generation and micro-fractures has been made by cutting glass using thermally generated stresses and strains. In this method a heat source, which is directed to the glass is moved with a fixed speed over the glass and thus produces comparatively large thermal stresses and strains so that cracks are formed in the glass. Infrared radiation sources, special gas burners and especially lasers have the required heat source properties to be able to position thermal energy locally, i.e. with a precision of better than a millimeter, which corresponds to a typical required cutting precision. Because of the ability to focus the laser beam on the glass, the applied power and distribution of the laser radiation in and through the glass can be controlled satisfactorily.
A method for scratching glass is disclosed in DE-AS 1 244 346 in which the glass is heated along a cutting path by laser radiation, while the temperature is kept under the melting temperature of the glass. After the heating the glass is cooled and cut by bending or impact. Also it can be heated over the melting temperature so that fine cracks or fractures are melted out.
GB-PS 1,433,563 describes a method in which the glass is worked with two laser beams. One of the beams has a comparatively lower energy and is used for pre-heating.
A process is described in DE 4 411 037 C2 in which a moving stress zone with a temperature of 250.degree. C. is produced in a hollow glass by means of a laser beam. A short starting scratch is produced mechanically by a short duration contact of a scratching point or tip with the surface of the hollow glass after the introduction of the stress zone, which is put substantially on the track or path exposed to the maximum laser beam intensity and thus which is at the highest temperature. The stress zone is cooled by means of a liquid absorbing fabric or web, whereby thermal shock and thus the stresses are increased so that the starting scratch forms a cut or fissure.
U.S. Pat. No. 5,237,150 describes a cutting process for thick steel plates, which uses a laser beam in a ring mode in order to protect the focusing lens. The laser beam is ring-like when it impinges on the lens, so that the energy of the laser beam is distributed over a comparatively larger surface of the lens than when a point-like laser beam impinges on the lens. Because of that the local heating of the lens material is avoided. The beam itself is however focused to a point or spot on the workpiece by means of the focusing lens. No increased intensity at the edge regions as in the case of a ring-like spot is present in the resulting spot, since the ring-like beam is brought to a "point", which cuts the steel along the cutting line as a "cutting point".
A laser beam in ring mode TEM.sub.o is similarly used, which is focused like a point on the workpiece in the method described in EP 0 062 484. When the laser beam is however focused like a point, the intensity maxima are merged at the point. Because of the laser beam is focused on the surface of the workpiece, the glass evaporates up to a certain depth. The remainder of the glass is heated above the melting point. The vaporized glass material is removed by means of a gas.
DE-OS 43 05 107 relates to a method and apparatus for cutting glass with a laser beam, whereby the laser beam is divided into two parallel beams which act on the glass symmetrically to the cutting line. No exact step (+0.1 mm) can be produced with this type of arrangement, but the cut weaves between both beam tracks.
The method described in WO 93/20015 uses a laser beam with an elliptical shape. This method produces good results with straight scratches or cracks in non-metallic plate material but no precise and accurate cut can be produced reliably along a curved path. Furthermore the stability of the cutting path in this process is poor at high radiation density and high cutting speed. This occurs because the heating with a laser beam has an elliptical cross section so that the Gaussian distribution of radiation density occurs in a very narrow range, whereby the temperature increases dramatically from the periphery to the center. It is extremely complicated to obtain a stable thermal fissure at high speed, with a deep scratch and yet with a stable power density, when the heating of the workpiece frequently results in overheating the central region of the irradiated portion of the workpiece, i.e. the softening temperature of the material is exceeded, although this is not permitted in precession cutting.
WO 96/20062 describes the closest state of the art method to that of the invention. This reference describes a method of cutting flat workpieces of brittle material, especially glass, along a predetermined cutting line with a heat radiation spot arranged symmetrically on the cutting line, which has an increased radiation intensity in its edge zones with a temperature maximum at its rear end, whereby the heated zone is moved along the separating line and/or the workpiece and in which the heated separated line section is subsequently cooled. These features are set forth in the preliminary portion of the main method claim appended hereinbelow.
In the known case an oval or elliptical heated region is provided with a radiation intensity minimum inside of the oval. This "cutting spot" intersections the separating line twice, namely at the front and at the rear end of the oval. Because of that however an unsatisfactory temperature distribution results, which is shown in FIG. 1 of the WO-paper. An unnecessary heating occurs already in the forward region of the elliptical cutting region that is forward in the cutting direction in the vicinity of the cutting line because of the forward cutting point.
An unnecessarily large heating arises in the center of the cutting region, i.e. on the cutting line, because of that so that at the end of the burning zone, where the laser beam intensity is very large in the vicinity of the cutting line and the temperature has reached a maximum, the glass can already be melted under the circumstances.
Furthermore in this method only glass with a thickness up to 0.2 millimeters can be cut, because with the higher required beam power a melting occurs and the cutting is interrupted. With glasses of grater thickness only cracking of the glasses occurs.